We are studying the relationships of biological function to three-dimensional geometrical and electronic structures of molecules. Recent efforts have been concentrated on the allosteric mechanisms of the regulatory enzyme, aspartate transcarbamylase. During this past year we have achieved a unique trace of both the regulatory (R) and catalytic (C) chains in the C6R6 molecule which has CTP (cytidine triphosphate) attached to each R chain. This solution to atomic resolution will be employed in the molecular replacement method for refining our 3 A structure of unliganded C6R6 and the 3.5 A three-dimensional data obtained during this past year on the form of the C6R6 molecule which has six molecules of PALA (N-phosphonacetyl-L-aspartate) in the active sites of the six catalytic units, R6 (C. PALA) 6. In addition, our theoretical studies of active site models of enzymes will be greatly facilitated by the extension now at hand, of our molecular orbital program to include d orbitals. Similar theoretical studies are under way on carcinogens and on large polyhedral boranes and carboranes.